Application analysis of grounding detection engine

Analysis of engineering application of grounding detection in small current grounding system

small current with ungrounded neutral or grounded through arc suppression coil is widely used in 6 ~ 66kV power system "In the more advanced current grounding system mode of automobile, when a single-phase grounding fault occurs in the system, because it cannot form a short working circuit for necessary cleaning and drying treatment, the grounding fault current is often very small, and the symmetry of the system line voltage is not destroyed. The system can continue to operate for a period of time, which is generally 1 ~ 2H according to the regulations. In order to prevent the expansion of the system accident, we must try to eliminate the grounding point during the grounding operation 。 Since the invention of the small current grounding line selection device in the 1980s, it has undergone several technical updates, and its line selection accuracy is also constantly improving. Although the standby manufacturer claims 100% line selection accuracy, there is a high misjudgment rate in engineering practice, which makes many users feel that it is not troublesome to use it, Therefore, many situations on site are that the selective inspection equipment is idle and exits, and the original method of manual switch off test is used to find the grounding

1 characteristics of single-phase grounding of small current system

analyze the operation state of single-phase grounding of small current system. The information different from the normal operation state mainly has two points: the zero sequence current flowing through the fault line is the capacitive current of the whole system minus its own capacitive current, and the zero sequence current flowing through the fault line is only the capacitive current of the line. The zero sequence current of the fault line flows from the line to the bus, while the zero sequence current of the non fault line flows from the bus to the line. The two directions are opposite, or the two are in reverse. Judging from the different state information between the single-phase grounding of the small current system and the normal operation, the judgment of the fault line seems to be very easy, but this is not the case. The main reasons are as follows:

1.1 the current signal is too small

the zero sequence current generated by the single-phase grounding of the small current system is the system capacitive current, which is related to the size of the system and the type of line (cable or overhead line), and the value is very small, After the neutral point is connected to the arc suppression coil for compensation, its value is smaller, and the compensation state of the arc suppression coil (over compensation, under compensation, complete compensation) is different. The characteristics of the grounding fundamental capacitive current are opposite or the same as those without arc suppression coil compensation. For the small current system with arc suppression coil, the 5th harmonic current or the active power direction of zero sequence current is used to detect, and the 5th harmonic current is 20 ~ 50 times smaller than the zero sequence current

1.2 large interference and low signal-to-noise ratio

interference in small current system mainly includes two aspects: first, at the installation site of single-phase grounding protection device in small current system of substation and power plant, electromagnetic interference is large; Second, the zero sequence current and harmonic current caused by the unbalanced load current are large, especially when the system is small and the capacitive current to the ground is small, the zero sequence current and harmonic current of the grounding circuit are even smaller than the corresponding current of the non grounding circuit

1.3 uncertainty affected by random factors

distribution in China is generally a small current system, and its operation mode changes frequently, resulting in frequent changes in the length and number of outgoing lines in substations, as well as frequent changes in capacitive current and harmonic current; In addition, the bus voltage level and the load current are always changing; The grounding resistance of the fault point is uncertain, etc. These have caused the instability of zero sequence fault capacitive current and zero sequence harmonic current

1.4 instability of capacitance current waveform

single phase grounding fault of small current system is often intermittent unstable arc grounding, so the capacitance current waveform is unstable, and the corresponding harmonic current is changing at any time

2 classification of grounding signal devices in small current grounding system

based on the characteristics of single-phase grounding in small current grounding system, at present, the design criteria of small current grounding signal devices mainly include the following 8 kinds: ① reflect the size of zero sequence voltage; ② Reflect the magnitude of power frequency capacitance current; ③ Reflect the direction of loose current in power frequency capacitance; ④ Reflect the active component of zero sequence current; ⑤ It reflects that the world consumption was less than 1000 tons of harmonic components five times before 1968 when grounding; ⑥ Reflect the first half wave of transient component of grounding fault current; ⑦ Signal injection method; ⑧ Group amplitude and phase comparison method